1. Field of the Invention
The present invention relates to an improved surface acoustic wave (SAW) device which comprises ZnO and diamond. The SAW device according to the present invention is applicable in operating at higher frequency.
2. Related Background Art
A surface acoustic wave device (SAW device) is a device which utilizes the acoustic wave propagation and the piezoelectricity on the surface of particular solid materials. The SAW device has excellent temperature stability, durability, and phase characteristics. Thus, there are certain demands, in the field of the advanced communication technology, for SAW devices which can be used in high frequency bands of more than 2 GHz, such as band pass filters, resonators, delay devices, signal processing devices, convolvers, and functional elements for opto-electronic devices. For example, the band pass filter with wider bandwidth is necessary for the equipments for cellular phones/communications which are typically operated in high frequency bands of more than 2 GHz.
The SAW devices typically comprises interdigital transducers (IDT) for generating and detecting the surface acoustic wave. The operation frequency (f) of a SAW device is given by the equation: f=V/.lambda., where V is the wave propagation velocity in the SAW device, and .lambda. is the wavelength of the surface acoustic wave. The operation frequency of more than 2 GHz is required for SAW device to be used for the band pass filters with wider bandwidth. The wavelength .lambda. is generally proportional to the width (d) of electrodes of the interdigital transducer.
Because of difficulties on micro-fabrication technique, the electrode with the width (d) of less than 0.5 .mu.m is difficult to be obtained, thus it is difficult to achieve desirable operation frequency (f) of more than 2 GHz by decreasing wavelength .lambda.. Therefore, the SAW device with higher propagation velocity V is required for being applied to the operation at the frequency of 2 GHz or higher.
The energy transform (piezoelectric) efficiency is also important for SAW device to be used for the advanced communication equipments. The effective coupling coefficient (K.sup.2) is an index to conversion efficiency of the converting of electrical energy into mechanical energy on the surface of the device. The preferable range of the effective coupling coefficient depends upon applications; about 0.10%--about 0.7% for narrow-band filter; about 0.7%--about 3% for medium-band filter; and about 3%--about 6% for wide-band filter.
The temperature coefficient of frequency (TCF) of the SAW device is preferable to be small as possible, because the smaller temperature dependency of the SAW device is desirable. The propagation loss of the SAW device is also preferable to be small as possible, since smaller attenuation in propagation of surface acoustic wave is desirable.
Japanese Patent Laid-Open No. 01-339,521 discloses SAW devices comprising a ZnO piezoelectric layer formed onto a diamond layer. Another type of SAW devices comprising LiNbO.sub.3 piezoelectric layer is disclosed in Japanese Patent Laid-Open No. 08-32,398. Japanese Patent Laid-Open 08-65,088 discloses SAW devices comprising LiTaO.sub.3 piezoelectric layer.
FIGS. 6A to 6G illustrate the layer/electrode constitution of SAW devices. In U.S. Pat. No. 5,446,329 to Nakahata et. al., disclosures of which is incorporated by reference, propagation velocity V, effective coupling coefficient K.sup.2 and temperature coefficient with frequency TCF of ZnO-diamond SAW devices have been improved, in particular by focusing first mode surface acoustic wave: for example, V of 8,000 to 10,000 (m/s), TCF of -10 to 10 (ppm/.degree. C.) and K.sup.2 of 0.7 to 1.7 (%) are achieved for "type E" constitution shown in FIG. 6E; V of 8,000 to 10,000 (m/s), TCF of -10 to 10 (ppm/.degree. C.) and K.sup.2 of 1 to 3 (%) are achieved for "type B" constitution shown in FIG. 6B; V of 8,000 to 10,000 (m/s), TCF of -10 to 10 (ppm/.degree. C.) and K.sup.2 of 1.5 to 4.5 (%) are achieved for "type F" constitution shown in FIG. 6F; V of 8,000 to 10,000 (m/s), TCF of -10 to 10 (ppm/.degree. C.) and K.sup.2 of 0.8 to 2.3 (%) are achieved for "type D" constitution shown in FIG. 6D; and V of 8,000 to 10,000 (m/s), TCF of -10 to 10 (ppm/.degree. C.) and K.sup.2 of 0.7 to 2.2 (%) are achieved for "type G" constitution shown in FIG. 6G.
It is also known that the performances of the SAW device can be further improved by employing LiNbO.sub.3 or LiTaO.sub.3 for piezoelectric material of SAW device. In the Japanese Patent Laid-Open No. 08-32,398, it is demonstrated that LiNbO.sub.3 -diamond SAW devices have V of 11,000 to 12,500 (m/sec.), TCF of -10 to 10 (ppm), and K.sup.2 of 7.5 to 9.5 (%).
Nevertheless, the use of ZnO for piezoelectric material of SAW device can provide significant advantage in fabricating SAW device, because the processibility of ZnO film is much better than LiNbO.sub.3 or LiTaO.sub.3 films. Therefore, it is desirable to further improve the performances of SAW devices which comprise ZnO piezoelectric layer.
It is also desirable to further improve the performances of the SAW devices in which an electrode, typically made of aluminum (Al), is not included between diamond layer and ZnO layer, such as "type A" and "type C" devices shown in FIG. 6A and FIG. 6C, respectively. Because such device constitution can eliminate the limitation on the process conditions for forming ZnO layer such as process temperature, since Al electrode having relatively low melting point is not included.
Therefore, it is an object of the invention to further improve the propagation velocity V, the effective coupling coefficient K.sup.2, the thermal coefficient of frequency TCF and propagation loss of the SAW device which includes ZnO piezoelectric layer formed on diamond layer, to provide SAW device having improved operation characteristics at the frequency of 2 GHZ or higher with superior durability and less energy loss.